def low_accuracy_recompute_alpha_varying_fields(
sph_start, sph_end, t_start, t_end, mag_params):
"""
Compute effective damping from change in magnetisation and change in
applied field.
From Nonlinear magnetization dynamics in nanosystems eqn (2.15).
See notes 30/7/13.
Derivatives are estimated using BDF1 finite differences.
"""
# Only for normalised problems!
assert(mag_params.Ms == 1)
# Get some values
dt = t_end - t_start
m_cart_end = utils.sph2cart(sph_end)
h_eff_end = heff(mag_params, t_end, m_cart_end)
mxh = sp.cross(m_cart_end, h_eff_end)
# Finite difference derivatives
dhadt = (mag_params.Hvec(t_start) - mag_params.Hvec(t_end))/dt
assert(all(dhadt == 0)) # no field for now
dedt = (llg_state_energy(sph_end, mag_params, t_end)
- llg_state_energy(sph_start, mag_params, t_start)
)/dt
sigma = sp.dot(mxh, mxh) / (dedt + sp.dot(m_cart_end, dhadt))
possible_alphas = sp.roots([1, sigma, 1])
a = (-sigma + sqrt(sigma**2 - 4))/2
b = (-sigma - sqrt(sigma**2 - 4))/2
possible_alphas2 = [a, b]
utils.assert_list_almost_equal(possible_alphas, possible_alphas2)
print(sigma, possible_alphas)
def real_and_positive(x):
return sp.isreal(x) and x > 0
alphas = filter(real_and_positive, possible_alphas)
assert(len(alphas) == 1)
return sp.real(alphas[0])
def recompute_alpha_varying_fields_at_midpoint(sph_start, sph_end,
t_start, t_end, mag_params):
"""
Compute effective damping from change in magnetisation and change in
applied field.
See notes 30/7/13 pg 5.
Derivatives are estimated using midpoint method finite differences, all
values are computed at the midpoint (m = (m_n + m_n-1)/2, similarly for
t).
"""
# Only for normalised problems!
assert(mag_params.Ms == 1)
# Get some values
dt = t_end - t_start
t = (t_end + t_start)/2
m = (sp.array(utils.sph2cart(sph_end))
+ sp.array(utils.sph2cart(sph_start)))/2
h_eff = heff(mag_params, t, m)
mxh = sp.cross(m, h_eff)
# Finite difference derivatives
dhadt = (mag_params.Hvec(t_end) - mag_params.Hvec(t_start))/dt
dedt = (llg_state_energy(sph_end, mag_params, t_end)
- llg_state_energy(sph_start, mag_params, t_start)
)/dt
dmdt = (sp.array(utils.sph2cart(sph_end))
- sp.array(utils.sph2cart(sph_start)))/dt
# utils.assert_almost_equal(dedt, sp.dot(m_cart_end, dhadt)
# + sp.dot(dmdt, h_eff_end), 1e-2)
# print(sp.dot(m_cart_end, dhadt), dedt)
# Calculate alpha itself using the forumla derived in notes
alpha = -((dedt + sp.dot(m, dhadt))
/ (sp.dot(h_eff, sp.cross(m, dmdt))))
return alpha
def recompute_alpha_varying_fields(
sph_start, sph_end, t_start, t_end, mag_params):
"""
Compute effective damping from change in magnetisation and change in
applied field.
See notes 30/7/13 pg 5.
Derivatives are estimated using BDF1 finite differences.
"""
# Only for normalised problems!
assert(mag_params.Ms == 1)
# Get some values
dt = t_end - t_start
m_cart_end = utils.sph2cart(sph_end)
h_eff_end = heff(mag_params, t_end, m_cart_end)
mxh = sp.cross(m_cart_end, h_eff_end)
# Finite difference derivatives
dhadt = (mag_params.Hvec(t_start) - mag_params.Hvec(t_end))/dt
dedt = (llg_state_energy(sph_end, mag_params, t_end)
- llg_state_energy(sph_start, mag_params, t_start)
)/dt
dmdt = (sp.array(utils.sph2cart(sph_start))
- sp.array(m_cart_end))/dt
utils.assert_almost_equal(dedt, sp.dot(m_cart_end, dhadt)
+ sp.dot(dmdt, h_eff_end), 1e-2)
# print(sp.dot(m_cart_end, dhadt), dedt)
# Calculate alpha itself using the forumla derived in notes
alpha = ((dedt - sp.dot(m_cart_end, dhadt))
/ (sp.dot(h_eff_end, sp.cross(m_cart_end, dmdt))))
return alpha
